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1 Dipartimento di Scienze della Terra "A.Desio," Università degli Studi di Milano, via Mangiagalli, 34, 20133 Milano, Italy
2 R 32 du Centre National de la Recherche Scientifique, Micropaléontologie, case 104, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris 05, France
3 Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA
Biotic changes in nannofossils and radiolarians associated with the Valanginian 
13C anomaly are documented at Ocean Drilling Program Hole 1149B in the Pacific Ocean: they are coeval and similar to those previously documented in the Tethys, suggesting a global perturbation of marine ecosystems. A marked increase in abundance of Diazomatolithus, absence of nannoconids, and a Pantanellium peak characterize the Valanginian 13C excursion. Such changes are interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO2. The occurrence of organic C–rich black shales in the Southern Alps and in the Pacific in the interval corresponding to the 13C excursion suggests a Valanginian oceanic anoxic event (OAE). Volcanism of the Paranà-Etendeka large igneous province (ca. 132 Ma) was presumably responsible for an increase of CO2, triggering a climate change and accelerated hydrological cycling, possibly causing an indirect fertilization of the oceans. Widespread nutrification via introduction of biolimiting metals at spreading ridges could have significantly increased during the Gondwana breakup and simultaneous tectonic events in three separate oceans. There is no paleontological or 18O evidence of warming during the Valanginian OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the 13C excursion. Weathering of basalts and burial of organic C–rich black shales were presumably responsible for CO2 drawdown and establishment of reversed greenhouse conditions.
Key Words: Valanginian • oceanic anoxic event • planktonic communities • carbon isotopes
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